I. Field of the Invention
This invention relates to the cooling and guiding of casting belts in apparatus used for continuously casting metal strip articles, particularly twin-belt casters used for casting aluminum alloys and similar metals. The invention also relates to belt casting apparatus incorporating such cooling and guiding equipment.
II. Background Art
The production of metal strip articles, particularly those made of aluminum and aluminum alloys, by twin-belt casting is well known in the art. Casting of this kind involves the use of a pair of endless belts, usually made of flexible but stiffly resilient steel, copper, or the like, which are rotatably driven over appropriate rollers and other path-defining means and supports. The belts define a casting mold formed between moving casting surfaces of confronting generally planar sections of the belts. Molten metal is continuously introduced into the inlet end of the mold via an injector or other feed device, and the metal is cooled as it passes through the mold, to emerge as a continuous metal strip article of desired thickness. A cooling apparatus is generally provided for each belt to provide the necessary cooling effect to cause metal solidification in the mold. Such cooling apparatus may operate by applying a liquid cooling liquid (e.g. water or water with appropriate additives) to the reverse surface of each belt, i.e. the surface opposite to the casting surface in the region of the casting mold, and then withdrawing, and usually recycling, the cooling liquid after it has provided the desired cooling effect. It is also usual in apparatus of this kind to apply a liquid belt dressing, e.g. oil or the like, to the casting surface of each belt before it enters the casting mold. This helps to control the rate of heat transfer from the molten metal to the belts and prevents the molten metal from bonding to the belts.
Twin belt casting apparatus of this kind is disclosed, for example, in U.S. Pat. No. 4,008,750 which issued on Feb. 22, 1977 to Sivilotti et al, U.S. Pat. No. 4,061,178 which issued on Dec. 6, 1977 to Sivilotti et al, U.S. Pat. No. 4,061,177 which issued on Dec. 6, 1977 to Sivilotti and U.S. Pat. No. 4,193,440 which issued on Mar. 18, 1980 to Thorburn et al. The teaching of these patents is specifically incorporated herein by reference. The '440 patent discloses an arrangement of belt cooling and guiding means that include generally planar supports for the belts made up of an array of spring-loaded cooling nozzles having hexagonal faces provided with central orifices, from which a cooling liquid is caused to flow under pressure into contact with the reverse surfaces of the belts as they pass through the casting mold. The hexagonal shape of the nozzles means that they may be arranged closely adjacent to each other to form a virtually continuous surface to provide both good support and even cooling effects. However, the nozzles are not quite contiguous so that small gaps remain through which the spent cooling liquid can be drawn under suction from below. The arrangement not only provides cooling, but also helps to hold the belts to the underlying supports by virtue of the vacuum created beneath the belts by the suction means used to withdraw the cooling liquid.
While the above apparatus has proved to be very effective, difficulties have emerged, particularly when apparatus of this kind is used to produce thinner strip articles than those produced conventionally (e.g. strip articles having a thickness in the range of 4 to 10 mm, compared to 10 to 30 mm for conventional castings), and/or those made from alloys having longer freezing ranges (e.g. those having a freezing range of 40 to 50° C., compared to up to 20° C. for alloys of shorter freezing range). Alloys of long freezing range must be cooled much more quickly and uniformly than alloys of short freezing range to achieve good surface and internal quality plus solidification within the mold. Strip articles of this reduced thickness, and articles made of alloys having longer freezing ranges, are of particular interest to the automotive industry. However, the casting of these alloys and thicknesses requires more controlled casting conditions than can be provided by previous casting cooling systems.
Accordingly, there is a need for improved belt cooling and guiding apparatus and methods so that these problems may be avoided during the use of twin-belt casting apparatus.